Engine combustion may generate particulate matter (PM) (such as soot and aerosols) that can be exhausted to the atmosphere. To enable emissions compliance, particulate matter filters, such as diesel particulate filters (DPFs) or gasoline particulate filters (GPFs) may be included in the engine exhaust, to filter out exhaust PMs before releasing the exhaust to the atmosphere. In addition, one or more soot sensors may be used to diagnose the DPFs and such soot sensors may be coupled upstream and/or downstream of the DPF.
As such, various types of soot sensors have been developed to sense soot production and release. One example approach shown by Paterson in U.S. Pat. No. 8,310,249 discloses soot sensors that collect particulate matter on charged electrodes. The soot sensor comprises opposed electrodes separated by an insulator with a gap in between to prevent current flow. When soot particles start to accumulate on the sensor, a bridge is created between the electrodes allowing current to flow. The change in current is used as an indication for soot deposition. In addition to electrode-based sensors, pressure-based soot sensors have also been developed. For example, as described by Sun et al. in U.S. Pat. No. 8,209,962, differential pressure across a particulate filter may be used for monitoring filter performance. Therein, when the differential pressure is less than a threshold, a leak in the particulate filter may be determined.
However, the inventors herein have recognized potential disadvantages with the above approaches. As one example, non-uniform or low soot deposit on the surface can occur due to biased flow distribution across the sensor surface, resulting in inaccurate voltage and current readings across the gap. Additionally, it may be difficult to reach sensor regeneration temperatures due to large flow impingement on the surface in some sensor designs. Further still, the sensors may become contaminated due to impingement of large diesel particulates or water droplets on the surface of sensor electrodes. Contamination of the sensor and interference in sensor results may also be caused by the large diesel particulates or water droplets infiltrating into the inner protection tube of the sensors. The inventors herein have identified an approach by which the issues described above may be at least partly addressed.
One example method includes, flowing exhaust gas from downstream of a first filter towards each of a first pressure sensor coupled at a first location in an exhaust pipe and a second pressure sensor coupled at a second location in a passage external to the exhaust pipe, the passage including a second filter coupled to an electric circuit, and indicating degradation of the first filter based on an interval between successive regenerations of the second filter. The first filter may be a diesel or gasoline particulate matter filter having a first, higher soot capacity, and the second filter may be a metal filter having a second, lower soot capacity. In this way, DPF diagnostics may be performed with higher accuracy and reliability without the results being corrupted by flow and soot loading distribution or impingement of water droplets.
As an example, exhaust gas may be diverted from a main exhaust pipe, downstream of a DPF, into an exhaust bypass parallel to the main exhaust pipe, outside of the main exhaust pipe via an inlet pipe. The inlet pipe may include perforations that allow water droplets and aggregated particulates to be trapped and released into the tailpipe. Downstream of the inlet pipe, the exhaust passage may be fitted with a first pressure sensor. In addition, the exhaust bypass passage may also be fitted with a second pressure sensor, downstream of a metal particle filter (MPF) coupled to an electric circuit, coupled to the exhaust bypass passage. After passing through the MPF, exhaust is returned to the main exhaust pipe via an outlet pipe. As exhaust gas diverted from the main exhaust pipe is received in the exhaust bypass, exhaust particulate materials, such as soot, may be deposited on the MPF therein, while exhaust containing soot flows unobstructed through the exhaust pipe towards the first pressure sensor. Exhaust pressure difference is calculated based on output from pressure sensors measuring pressure at the exhaust pipe and the exhaust bypass. The pressure difference between a second pressure sensor in the exhaust bypass, downstream of the MPF, and the first pressure sensor in the exhaust pipe may be used to infer a soot loading of the MPF upstream of the second pressure sensor and initiate regeneration of the MPF by closing the electric circuit coupled thereto. Further, a time interval elapsed between successive regenerations of the MPF may be monitored. As such, if the DPF in the exhaust pipe becomes degraded (such as due to age or durability issues), an increasing amount of soot may escape from the DPF, and travel onto the MPF. As a result, the MPF may have to be cleaned more frequently. Thus, based on a decrease in the time interval between successive regenerations of the metal filter in the exhaust bypass, degradation of an upstream DPF may be determined, and appropriate actions may be taken.
In this way, by diverting a portion of exhaust gas from an exhaust pipe to a soot sensor with a metal filter, located downstream of a diesel particulate filter, degradation of a particulate filter can be detected based on an amount of soot leaking from the particulate filter onto the metal filter. The technical effect of trapping soot particles on the metal filter selectively included in the exhaust bypass is that a pressure difference of exhaust between the second location at the exhaust bypass and the first location in the main exhaust pipe can be advantageously used to learn the soot load of the metal filter. As such, this reduces the need for multiple sensors for soot load estimation. The technical effect of trapping aggregated particulates and water droplets in an inlet pipe of the soot sensor, and redirecting them to the exhaust tailpipe, is that impingement of aggregated particulates and water droplets on the soot sensor is reduced, allowing for more accurate and reliable soot detection. By relying on a time interval between successive regenerations of the metal filter to detect DPF degradation, is that the diagnostics may be rendered more sensitive and less affected by variations in soot loading distribution on the metal filter. Overall, accuracy and reliability of soot sensing and diagnosing of an exhaust particulate filter is increased, enabling higher emissions compliance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.